Prior trailers, as typified in U.S. Pat. No. 2,466,102, attempted to lower the trailer bed to ground level, but were only able to accomplish this through an elaborate wheel suspension system. Through the use of winches and pulleys, the entire trailer structure was lowered to ground level, rather than just the bed itself.
Use of a pivoting lever frame, as illustrated in U.S. Pat. No. 3,860,255, allowed the rear of the trailer to lower, but resulted in the front portion of the outside frame to rise to an undesired height, since the pivot point was situated directly behind the wheel axles, and forward of the end of the trailer. To give the necessary weight to the rear portion of the trailer, the load bed extended some distance beyond the frame, or substantial mass had to be added to the rear portion.
A subsequent trailer, as illustrated in U.S. Pat. No. 4,077,643, had a front tongue frame that was hinged to the front end of a trailer bed, with the trailer bed lowering toward the ground through the use of small pivot arms attached to the trailer bed, rotating about a drop style wheel axle. The small pivot arms failed to provide a substantial amount of leverage, and there was rotation of the wheels during loading and unloading.
A motorcycle trailer, as illustrated in U.S. Pat. No. 4,659,100, employed a see-saw bar as a lever to raise and lower the rear portion of the trailer. The front portion of the trailer did not lower, and the trailer functioned merely as an inclined ramp during the loading process.